ABSTRACT: Particulate organic carbon (POC) is an important component in the carbon cycle of estuarine systems. Many studies have utilized carbon isotopic composition (d13C-POC) to estimate the origin of POC. Most of the early studies were restricted to areas of minimum terrestrial inputs of POC, whereas later studies used multiple stable isotope analysis. In our study, we assess the proportion of POC from C3 and C4 plants and microalgae in two terrestrially affected, contrasting estuaries of the southeastern U.S.A.: the Altamaha (Piedmont) and the Satilla (blackwater) Rivers. Our analysis was based on a mass balance equation, taking into consideration the estimated amounts of microalgae and C3 and C4 plant material. Analysis of d13C of extracted chlorophyll a (Chl a) was used to estimate the isotopic signature of microalgae to constrain the three endmember mass balance. The maximum concentration of POC fluctuated between 9 and 11 mg C L-1, coinciding with the period of maximum riverine flow. The POC represented 50-70% of the total organic carbon. However, large fluctuations were observed, suggesting important tidal- and wind-driven resuspension. The d13C-POC ranged from -21 to -28‰, being more depleted at lower salinities. This indicates rapid dilution of terrestrially derived material with estuarine- or marine-derived material. The Chl a concentration was maximal during summer in upstream stations of the Altamaha estuary, while the Satilla generally showed a midestuary maximum. The d13C of Chl a ranged from -20.7 to -31.43‰, indicating isotopically depleted inputs of microalgal-derived material (MaDM). Our results suggest that MaDM is the dominant component of POC in the Altamaha River during summer, whereas the C3 component is dominant during periods of lower productivity and high flushing. Although MaDM is significant in the Satilla estuary, most of the POC consists of material derived from C3 and C4 plants.

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